The present invention relates to polysilicon thin film transistors, and in particular, it relates to those thin film transistors that are suitable for use in fabricating radiation sensors and flat panel displays.
Field Effect Thin Film Transistors (TFTs) fabricated from polysilicon are becoming important due to their potential for use in sensors and flat panel displays. Polysilicon is becoming a material of choice since TFTs fabricated from polysilicon offer high mobility and stable operation.
However, grain boundaries in polysilicon TFTs exert considerable influence on TFT characteristics such as degradation of carrier transport. Polysilicon TFTs have an anomalous leakage current which decreases the ON/OFF current ratio of the drain current. One source of leakage current is the leakage current produced by grain-boundary defects. Trap states at the grain boundaries result in a localized potential barrier being formed for passage of carriers from grain to grain. Therefore, reducing the trap state density becomes increasingly important since it has been suggested that decreasing the grain boundary trap state density can reduce the leakage current and enhance mobility.
Hydrogen passivation is one known method for reducing grain boundary trap density and improving the performance of polysilicon TFTs. Kamins et al, "Hydrogenation of Transistors Fabricated in Polycrystalline-Silicon Films" IEEE Electron Device Letters, Volume EDL-1, No. 8, pgs. 159-161 (August 1980) discussed the effect of subjecting a completed polysilicon thin film transistor to a hydrogen-nitrogen plasma in a planar plasma reactor. The plasma creates an active species of hydrogen which can then migrate into the polysilicon film, passivating the grain boundary states. After hydrogenation, field effect mobility increased by more than a factor of 10 from 2.6 to 34 cm.sup.2 /V-sec. Proano et al, "Fabrication and Properties of Single, Double, and Triple Gate Polycrystalline-Silicon Thin Film Transistors", Proc. of Materials Research Society Symposium, Vol. 106, pgs. 317-322 (1988), discussed the effect of plasma hydrogenation of polysilicon TFTs. The hydrogenation was performed in a hydrogen-nitrogen atmosphere at 300.degree. C. for up to 30 minutes. It was found that drain-source OFF current was reduced from 5.times.10.sup.-10 A to 1.times.10.sup.-12 A.
Takashi et al, "High-Performance Poly-Si TFT's With ECR-Plasma Hydrogen Passivation", IEEE Transactions on Electron Devices, Vol. 36, No. 3, pgs. 529-553 (March 1989), describe hydrogenation carried out in an electron cyclotron resonance reactor on polysilicon TFTs. A small grain polysilicon film (average grain size approximately 40 nm) was deposited on a quartz substrate at a thickness of 0.3 .mu.m by low pressure chemical vapor deposition at 625.degree. C. Recrystallization using CW-Ar laser annealing was performed on the polysilicon film to obtain large grains. A standard MOS transistor fabrication process was used to form the polysilicon TFTs. Gate oxide film was obtained by thermal-oxidation of polysilicon at 1,000.degree. C. in dry oxygen with a thickness of the oxide film at 0.1 .mu.m. At the gate electrode, a 0.3 .mu.m thick polysilicon layer was deposited by low pressure chemical vapor deposition and patterned. After deposition of the polysilicon layer, the source and drain regions were opened. A phosphosilicate glass (PSG) was deposited and annealed at 900.degree. C. for one hour in nitrogen to form n.sup.+ regions. The PSG film was removed and aluminum film deposited and patterned. The TFTs were then annealed in a hydrogen atmosphere at 400.degree. C. The TFTs were treated in a hydrogen plasma with ECR-plasma apparatus at 300.degree. C. A magnetron was used to generate microwaves at a frequency of 2.5 GHz through a rectangular wave guide into the plasma chamber. Argon was introduced into the plasma chamber at 15 sccm and hydrogen was introduced into the specimen chamber also at 15 sccm. Treatment was performed at 600 W for 30 minutes. After annealing, the carrier mobility was found to have increased from 33.7 cm.sup.2 /V. sec. to 151.0 cm.sup.2 /V. sec. It was also observed that the leakage current of the ECR hydrogen plasma treated TFTs was lower than that in the TFT without hydrogen passivation.
Pollack et al, "Hydrogen Passivation of Polysilicon MOSFET's From a Plasma Nitride Source", IEEE Electron Device Letters, Vol. EDL-5, No. 11, pgs. 468-470 (November 1984) studied the effects of hydrogen passivation of polysilicon TFTs using a plasma silicon nitride source. The hydrogen passivation was performed as an extra step with the hydrogen from silicon nitride source being driven into the polysilicon by annealing in nitrogen at 450.degree. C. A silicon nitride layer was plasma-deposited on the polysilicon TFT. The silicon nitride layer has sufficient hydrogen, typically greater than 10.sup.22 cm.sup.-3, such that the layer constitutes an ideal diffusion source for atomic hydrogen. Annealing was performed at 450.degree. C. to hydrogen passivate the polysilicon grain boundaries.
Faughnan et al, "A Study of Hydrogen Passivation of Grain Boundaries and Polysilicon Thin-Film Transistors", IEEE Transactions on Electron Devices, Vol. 36, No. 1, pgs. 101-107 (January 1989) discussed the effect of hydrogen passivation of grain boundaries on the leakage current of polycrystalline silicon thin-film transistors. Hydrogen passivation was carried out either by annealing the TFTs in a forming gas (5% hydrogen+95% ammonia) at 600.degree. C., 700.degree. C., and 800.degree. C., or by annealing in ammonia gas at 800.degree. C. followed by depositing a 60 nm layer of silicon nitride. Some wafers received further annealing in the forming gas at 500.degree. C. No further heat treatment was performed after the passivation step. Leakage current was found to be reduced but it was thought that the silicon nitride layer was too thin to play a role. It should be noted that the silicon nitride that was deposited in the study by Faughnan et al was deposited at 800.degree. C. At such a high temperature, hydrogen tends to be driven out of the deposited layer making the silicon nitride layer not a good hydrogen source.
The Meakin et al U.S. Pat. No. 4,880,753 describes a process for manufacturing a polysilicon TFT. The Troxell et al U.S. Pat. No. 4,851,363, the Nakagawa et al U.S. Pat. No. 4,766,477, the Pennell et al U.S. Pat. No. 4,751,196, and the Japanese published application 61-046069 describe the use of silicon nitride layers as insulating layers or surface protective layers against oxidation or moisture in TFTs.
In each of the above-mentioned examples optimizing the conditions to produce consistently high quality polysilicon TFTs has been difficult. For example, in hydrogen passivation of grain boundaries using an atomic hydrogen-containing layer such as silicon nitride films, the properties of the film are critical for obtaining effective and consistent hydrogen passivation of the grain boundaries. Furthermore, diffusion of the hydrogen from the silicon nitride layer occurs at deposition temperatures above 500.degree. C. See for instance Fritzsche, "Heterogeneities and Surface Effects in Glow Discharge Deposited Hydrogenated Amorphous Silicon Films", Thin Solid Films, Vol. 90, pgs. 119-129 (1982); Biegelsen et al, "Hydrogen Evolution and Defect Creation in Amorphous Si:H Alloys", Physical Review B, Vol. 20, No. 12, pgs. 4839-4846 (Dec. 15, 1979).